I get the dreaded P0400 code about every six months and its a ***** getting the pipe off each time. It binds. Maybe there IS a secret. Also, FYI, I can only get the rotorooter to go in 20" but it cures it so I guess that's the blockage
The reason that you have to do it every 6 months is because you are not going through 100% of the pipe.
I have always thought that it would be pretty easy to solder a "clean out" in the heel of the 90 bend at the manifold end that would let you have a straight shot into the manifold. The time to make the mod would be when you have the head off and you can put the pipe on the bench.